Enhanced graphitic domains of unreduced graphene oxide and the interplay of hydration behaviour and catalytic activity

Previous studies indicate that the properties of graphene oxide (GO) can be significantly improved by enhancing its graphitic domain size through thermal diffusion and clustering of functional groups. Remarkably, this transition takes place below the decomposition temperature of the functional groups and thus allows fine tuning of graphitic domains without compromising with the functionality of GO. By studying the transformation of GO under mild thermal treatment, we directly observe this size enhancement of graphitic domains from originally ≤40 nm² to >200 nm² through an extensive transmission electron microscopy (TEM) study. Additionally, we confirm the integrity of the functional groups during this process by a comprehensive chemical analysis. A closer look into the process confirms the theoretical predicted relevance for the room temperature stability of GO and the development of the composition of functional groups is explained with reaction pathways from theoretical calculations. We further investigate the influence of enlarged graphitic domains on the hydration behaviour of GO and the catalytic performance of single atom catalysts supported by GO. Additionally, we show that the sheet resistance of GO is reduced by several orders of magnitude during the mild thermal annealing process.     

Amelioration of mechanical brittleness in hyperbranched polymer. 1. Macroscopic evaluation by dynamic viscoelastic relaxation

Hyperbranched poly(ether ketone)-b-linear poly(ether ketone)-b-hyperbranched poly(ether ketone) (HLHPEK) triblock copolymers with two different block compositions were prepared as an approach to improve the mechanical brittleness of hyperbranched poly(ether ketone) (HPEK). From the time-temperature superposition of dynamic shear moduli, G′(ω) and G″(ω), of HPEK and two HLHPEKs, it was investigated that the junction points between G′(ω) and G″(ω) shifted to the higher frequencies and the rubbery plateau region spread wider over the reduced frequency axis as the linear blocks were introduced and their compositions were increased. Such changes in viscoelastic response were consequences of increase in the amount of chain entanglements additionally formed by the linear blocks. In order to verify the effect of the linear block incorporation on amelioration of the mechanical brittleness, the degree of brittleness and its improvement were evaluated from the temperature dependence of the shift factors, a_Ts, experimentally obtained during the superposition of the dynamic moduli and of the average viscoelastic relaxation times, τ_HNs, determined with empirical Havriliak-Negami distribution function. From the nonlinear curve fittings of the a_Ts and the τ_HNs by the Vogel-Tamman-Fulcher equation, the degree of brittleness for HPEK and HLHPEK triblock copolymers were quantified as values of the material parameter D, an indicative of deviation from the linear Arrhenius behavior and a measure of fragility of the given material. The tendency of increasing D values with the linear block compositions confirmed substantial improvement of the mechanical brittleness in the HLHPEK triblock copolymers compared to HPEK. Therefore, the approach to copolymerize HPEK with its chemically analogous linear counterpart was verified to be an effective strategy to impart molecular entanglements and hence ameliorate the mechanical brittleness on the basis of macroscopic rheological evaluation.     

Gaseous Nanocarving‐Mediated Carbon Framework with Spontaneous Metal Assembly for Structure‐Tunable Metal/Carbon Nanofibers

Vapor phase carbon (C)‐reduction‐based syntheses of C nanotubes and graphene, which are highly functional solid C nanomaterials, have received extensive attention in the field of materials science. This study suggests a revolutionary method for precisely controlling the C structures by oxidizing solid C nanomaterials into gaseous products in the opposite manner of the conventional approach. This gaseous nanocarving enables the modulation of inherent metal assembly in metal/C hybrid nanomaterials because of the promoted C oxidation at the metal/C interface, which produces inner pores inside C nanomaterials. This phenomenon is revealed by investigating the aspects of structure formation with selective C oxidation in the metal/C nanofibers, and density functional theory calculation. Interestingly, the tendency of C oxidation and calculated oxygen binding energy at the metal surface plane is coincident with the order Co > Ni > Cu > Pt. The customizable control of the structural factors of metal/C nanomaterials through thermodynamic‐calculation‐derived processing parameters is reported for the first time in this work. This approach can open a new class of gas–solid reaction‐based synthetic routes that dramatically broaden the structure‐design range of metal/C hybrid nanomaterials. It represents an advancement toward overcoming the limitations of intrinsic activities in various applications.     

Study on the thermo-effect of nugget growing in single-phase AC resistance spot welding based on the calculation of dynamic resistance

According to the welding current and electrode voltage detected in resistance spot welding, the dynamic resistance was calculated and the nugget growing thermo-effect in process of nugget forming was studied. The results showed that the process of nugget growth in resistance spot welding can be divided into three stages, which were initial stage, growing stage and stable stage. The welding current provided energy for nugget growth, which showed a high linear relationship with mean dynamic resistance and more prominent polynomial relationship with dynamic resistance heat. The dynamic resistance heat was an important evaluating indicator to the nugget growing thermo-effect, which was closely related to the characteristics of nugget forming. As the dynamic resistance heat was increased, the nugget diameter and tensile shear strength of spot weld were raised and showed a more prominent polynomial relationship. According to the calculation of dynamic resistance heat, these relationships provided a method for the nondestructive test of nugget quality features.     

Numerical approach of cyclic behaviour of 316LN stainless steel based on a polycrystal modelling including strain gradients

A non-local polycrystal approach, taking into account strain gradients, is proposed to simulate the 316LN stainless steel fatigue life curve in the hardening stage. Material parameters identification is performed on tensile curves corresponding to several 316LN polycrystals presenting different grain sizes. Applied to an actual 3D aggregate of 316LN stainless steel of 1200 grains, this model leads to an accurate prediction of cyclic curves. Geometrical Necessary Dislocation densities related to the computed strain gradient are added to the micro-plasticity laws. Compared to standard models, this model predicts a decrease of the local stresses as well as a grain size effect.     

Electrically assisted stress relief annealing of automotive springs

Journal of Mechanical Science and Technology - In the present study, Electrically assisted (EA) stress relief annealing for automotive springs is experimentally investigated. A concept of EA stress...     

Giant photoresponsivity of transfer free grown fluorographene – MoS2 heterostructured ultra-stable transistors

Development of metal dichalcogenides based air and water stable opto-electronic devices is a bottleneck in their commercial implementation. Here we address this issue by the direct catalyst-free deposition of fluorographene (FG) protective layer over monolayer MoS₂ (MS), where such an atomic interface is found to be providing enormous photoresponse and chemical stability to the device. Electric field modulated (ionic liquid (IL) electrolyte based top-gated) photodetectors are developed with MS only and FG-MS heterostructure, where the MS photodetector has the responsivity of ~1.3 A/W (with V_GS = 0 V while unstable with IL gating) while that of FG-MS is ~2000 A/W (with V_GS = 0 V and ~8000 A/W with V_GS = 1.5 V, and the detectivity ~10¹³). This giant photoresponse of the FG-MS is validated with the help of ultrafast transient absorption spectroscopy assisted charge carrier dynamics studies at different temperatures. The broad optical response (350–850 nm) of the FG-MS is found to be intact not only in IL based gating but also after exposing in water for a month or heat treated in air at 200 °C. Interfacing and capping with FG, developed via the direct growth method, are found to be ideal for realizing high shelf-life and good responsivity photodetectors and solar cells of several other monolayer TMDs too, while available for wafer-scale manufacture.     

Numerical simulation of gas-assisted polymer-melt electrospinning: Parametric study of a multinozzle system for mass production

In this study, we developed a multiphysics model for simulation of a gas-assisted melt-electrospinning (GAME) process, focusing on jet formation and propagation behavior. By numerically calculating the stresses acting on the jet during a single-nozzle GAME process, the shear viscous stress was identified as the main factor with respect to jet stretch; thus, the relationship between shear viscous stress and jet thickness was investigated. The jet stretch ratio increased sharply when shear viscous stress reached the level at which jet sharpening occurred, leading to stable jet formation. We defined this stress as the critical shear viscous stress to determine stable spinnability. By imposing an electric field distribution calculated for a multi-nozzle array (number of nozzles, tip-to-tip distance, and applied voltage) on the boundary condition of the single-nozzle GAME simulation model, multinozzle GAME was simulated; this enabled proposal of a spinnability diagram for stable spinning.     

Effect of ZnO/K2O ratio on the crystallization sequence and microstructure of lithium disilicate glass-ceramics

The effect of ZnO/K₂O (Z/K) ratio on the crystallization sequence and microstructure of lithium disilicate (Li₂Si₂O₅: LS2) glass-ceramics was carefully investigated for the SiO₂-Li₂O-K₂O-ZnO-P₂O₅ system. The Z/K ratios of precursor glasses were varied from 0 to 3.5 while the nucleating agent of P₂O₅ and glass modifiers of ZnO plus K₂O were fixed to have 1.5 and 4.5 mol% relative to LS2, respectively. For the samples prepared by two-stage heat treatments of 500 °C for 1 h and 800 °C for 2 h in air, the LS2 nucleation rate was increased with increasing the Z/K ratio due to the variation in crystallization sequence from type II (Li₂SiO₃: LS) to type I (LS + LS2) in addition to an amorphous phase separation in base glass. Consequently, with increasing the Z/K ratio, the LS2 crystalline phase within the glass matrix continuously changed from larger acicular ones to smaller equiaxed ones.